The present invention relates to a thick film temperature sensitive device, and more particularly to an electrical temperature sensing device of a vitreous enamel resistor type having a relatively high positive temperature coefficient of resistance, a relatively high resistivity, and a resistance to temperature characteristic which is highly linear, and a method and material for making the same.
In general, thick film temperature sensing devices of the vitreous enamel resistor type comprise a substrate having a film of glass and particles of a conductive material embedded in and dispersed throughout the glass film. The devices are made by first forming a mixture of a glass frit, and particles of the conductive material. The mixture is applied to substrates and fired at a temperature at which the glass frit softens. Certain vitreous resistors such as those utilizing precious metals and precious metal oxides are made by firing in an oxidizing atmosphere, while other vitreous resistors such as those using non precious metals, and non precious metal oxides, borides and nitrides, are formed by firing in a non-oxidizing environment. When cooled, the glass solidifies to form the resistors which have a glass film with the conductive particles therein.
In order to provide electrical connections to the device, it is desirable to provide a conductive termination at each end of their resistance films. Heretofore, as disclosed in U.S. Pat. No. 3,358,362 issued Dec. 19, 1967, terminations for vitreous enamel resistors have been provided by the electroless plating of a film of a metal, such as nickel or copper. However, it has been found that such electroless metal film terminations are not compatible with certain vitreous enamel resistance films. In order to make electrical connections to such resistor films, a precious metal, such as silver, is usually applied by another process.
The thick film temperature sensing device having metal conducting materials which have heretofore been produced, characteristically have relatively low temperature coefficients of resistance or low resistivities of less than 1 ohm/square. Where the device provides both relatively high temperature coefficients of resistance and resistivities greater than 1 ohm/square, as when iron metal particles alone are used as the conductor, the vitreous resistance film can not be processed by spiralling to provide a device with the desired resistance. In addition to providing relatively high temperature coefficients of resistance, it is also desirable that the coefficient be positive to provide current self limiting of the device, since in such case the resistance increases with an increase in current and the resulting rise in temperature. The high resistivity is also essential so that the device can be produced with a resistance sufficiently high to provide high sensitivity to small changes in temperature. A highly linear change in resistance with temperature is also desirable over a temperature range of -55.degree. C. to +150.degree. C. to provide accurate temperature indications without requiring special and costly compensating networks.